Measuring Flow Using a Permanent Magnet with a Large Constant Gradient

Nuclear magnetic resonance (NMR) has long been used for measuring flows, as it has the unique advantage of being a truly non-invasive method by which to do so. In this paper, we describe our use of a magnet with a large constant gradient to measure two decades of average velocities (0.002–0.7 m/s), principally using a measurement method developed by Osan et al. that uses the initial decay of a Carr–Purcell–Meiboom–Gill sequence to determine the mean flow velocity. Thanks to the repeatability afforded by the stable gradient, we reliably pinpoint the transition to turbulence for pipe flow through the magnet.